Since the polyester monofilament possesses a prominent shear strength, acid-resistance, and dimensional stability, it has found extensive utility in papermaking drier canvas, papermaking wire, various brushes, writing brushes, plain gauze for printing screens, and fishing lines, for example. When used under conditions susceptible to hydrolysis, because of a hot moist ambient air, as when employed as a component thread in a papermaking drier canvas, it gradually undergoes deterioration and a loss of tenacity with use, and at the same time, suffers an adhesion thereto and accumulation thereon of a stain comprising filling material, sizing agent, paper strength reinforcing agent and various other papermaking raw materials incorporated in the papermaking dope. Thus, this is disadvantageous in that it imposes restrictions as on the duration of safe use and the conditions of effective use thereof, and compels the canvas to be cleaned at frequent intervals. Various proposals have been made for ridding the polyester monofilament of such drawbacks as mentioned above.
As one example of the means of improving the resistance of the polyester monofilament to hydrolysis, a species of polyester monofilament having a specific amount of a polyolefin such as polyethylene, polypropylene, polybutene, poly-4-methyl pentene-1, or polystyrene incorporated in polyester is known (Japanese Unexamined Patent Publication No. 51-136923). The monofilament obtained by this particular technique, such as, for example, the monofilament made of a polyethylene-containing polyethylene terephthalate, exhibits a poor tenacity and a low resistance to hydrolysis, and therefore, lacks practicability.
Methods of improving the resistance of polyester to hydrolysis by adding a carbodiimide compound thereto are also known to the art. A method of forming a polyester filament containing no unaltered carbodiimide by adding a mono- or bis-carbodiimide compound to polyester, and quickly mixing them and spinning the resultant mixture (Japanese Unexamined Patent Publication No. 50-95517), a method which comprises adding a polycarbodiimide compound containing at least three carbodiimide radicals in the molecular unit thereof (Japanese Examined Patent Publication No. 38-15220), a polyester monofilament for a papermaking canvas having a specific amount of a specific carbodiimide compound retained in an unaltered form in polyester (Japanese Unexamined Patent Publication No. 58-23916), and a method of producing an industrial grade polyester filament having a specific carbodiimide compound incorporated in a polyester containing a specific amount of phosphorus atoms (Japanese Unexamined Patent Publication No. 57-205518) have been proposed, for example.
Various techniques have been proposed for improving the resistance of polyester fibers to staining. A technique for treating a textile product with a fluorine-containing compound (Japanese Unexamined Patent Publication No. 52-5400 and Japanese Unexamined Patent Publication No. 58-46123), core-sheath composite fibers using a sheath of fluorine resin (Japanese Unexamined Patent Publication No. 53-31851), a composition containing a perfluoroalkylsulfonate (Japanese Unexamined Patent Publication No. 59-66449), fibers containing fluorine resin (Japanese Unexamined Patent Publication No. 62-238822), and fibers containing minute fluorine resin particles (Japanese Unexamined Patent Publication No. 2-26919) are known to the art, but these known techniques cannot simultaneously satisfy the required two properties i.e., a resistance to hydrolysis and proof against staining.
In recent years, the conditions under which the polyester monofilament is used, as for the papermaking drier canvas, have become very strict, and to enhance the productivity of paper and prevent degradation of the paper quality, the desirability of a polyester monofilament combining a still better resistance to hydrolysis with an outstanding proof against staining has become necessary.
An object of this invention is to provide a polyester monofilament which cosines an excellent resistance to hydrolysis with a proof against staining, and is usable for a papermaking drier canvas.